Electron paramagnet resonance (EPR) techniques and apparatus for performing EPR spectroscopy on a flowing fluid

ABSTRACT

Certain aspects of the present disclosure provide methods and apparatus for performing electron paramagnetic resonance (EPR) spectroscopy on a fluid from a flowing well, such as fluid from hydrocarbon recovery operations flowing in a downhole tubular, wellhead, or pipeline. One example method generally includes, for a first EPR iteration, performing a first frequency sweep of discrete electromagnetic frequencies on a cavity containing the fluid; determining first parameter values of reflected signals from the first frequency sweep; selecting a first discrete frequency corresponding to one of the first parameter values that is less than a threshold value; activating a first electromagnetic field in the fluid at the first discrete frequency; and while the first electromagnetic field is activated, performing a first DC magnetic field sweep to generate a first EPR spectrum.

CLAIM OF PRIORITY UNDER 35 U.S.C. § 119

This application is a continuation application of U.S. patentapplication Ser. No. 15/875,823 entitled “Electron ParamagneticResonance (EPR) Techniques and Apparatus for Performing EPR Spectroscopyon a Flowing Fluid,” filed Jan. 19, 2018, which claims priority to U.S.Provisional Patent Application No. 62/448,095, entitled “ElectronParamagnetic Resonance (EPR) Sensor Based on Automated Closed-LoopImpedance Matching” and filed Jan. 19, 2017, which are all incorporatedby reference herein in their entireties.

BACKGROUND Field of the Disclosure

The present disclosure generally relates to electron paramagneticresonance (EPR) and, more specifically, to utilizing EPR methods andsystems to detect the characteristics of materials, for example, inwells, pipelines, or formations, such as for flow assurance or logging.

Relevant Background

Electron paramagnetic resonance (EPR), also referred to as electron spinresonance (ESR), is a spectroscopic and imaging technique that iscapable of providing quantitative information regarding the presence andconcentration of a variety of paramagnetic species within a sample undertest. The valence electrons of a paramagnetic species possess unpairedspin angular momentum and, thus, have net magnetic moments that tend toalign along an externally applied magnetic field. This alignment processis known as paramagnetization. EPR is a measurement technique thatrelies on the external manipulation of the direction of this electronparamagnetization, also referred to as a net electronic magnetic moment.In a typical EPR experiment, a polarizing static magnetic field B₀ (alsoreferred to as a DC magnetic field) is applied to a sample to align themagnetic moments of the electrons along the direction of the magneticfield B₀. Then, a high-frequency oscillating magnetic field B₁, oftenreferred to as the transverse magnetic field or the radio frequency (RF)magnetic field, is applied along a direction that is perpendicular tothe polarizing field B₀. Usually, the oscillating field B₁ is generatedusing a microwave resonator (fed via a coil or a transmission line) andis designed to excite the unpaired electrons by driving transitionsbetween the different angular momentum states of the unpairedelectron(s).

EPR technology is based on the interaction of these electron spins withthe applied RF (e.g., microwave) electromagnetic fields in the presenceof the external static (DC) magnetic field. EPR data provides valuableinformation about electronic structures and spin interactions inparamagnetic materials. EPR has found wide-ranging applications invarious science and engineering technology areas, such as studyingchemicals involving free radicals or transition metal ions.

The major components of an EPR spectroscope are typically similar. Forexample, U.S. Pat. No. 4,803,624 to Pilbrow et al., entitled “ElectronSpin Resonance Spectrometer” and issued Feb. 7, 1989, teaches anelectron spin resonance spectrometer with a magnetic sweep from 0 to0.15 tesla (T) operating in the range of 1 to 5 GHz with a loop-gapresonator containing a cavity to hold a fluid sample. This spectrometeruses a circulator to measure the reflected microwave signal from theresonator, the same as in most commercially available EPR spectrometers.Microwave circuit components (e.g., an isolator, circulator, powerdividers, variable attenuator, and directional couplers) are arranged ina microwave bridge connected by microstrip transmission lines. Externalcomponents, such as the microwave source and loop-gap resonator, areconnected via SMA coaxial connectors. External magnetic fields can beprovided by permanent magnets, electromagnets, or a combination of both.

It is known that the resonant frequency of a fluid-filled cavity changesdepending on the fluid properties therein, as does the efficiency of thecoupling of the electromagnetic field to the cavity. The pertinentelectrical parameters of the cavity are its dielectric and conductivityproperties, which combine into an effective permeability according tothe formula ϵ+iσ/ω, where ϵ is the ratio of electrical displacementfield to electric field, σ is the conductivity, and e^(−iωt) is thevariation of the field in time (i.e., ω is the radial frequency, equalto 2πf where f is excitation frequency). The displacement field can beout of phase with the electric field, in which case ϵ can be viewed as acomplex number, or else the imaginary component of ϵ can be incorporatedinto the conductivity. In this text, and as is common in theelectromagnetic community, the term “permittivity” is used to refer toboth the complex value ϵ+iσ/ω, and also to just the dielectriccomponent, ϵ. The intended meaning will be clear to a person havingordinary skill in the art. The magnetic properties of the medium aregiven by the permeability μ, which is the ratio of magnetic fluxintensity B to the magnetic field intensity H. In air, the permeabilityis denoted μ₀. More generally one can write μ=μ₀(1+χ), where χ is termedthe “susceptibility.” The B and H fields may be out of phase, in whichcase μ and χ are also complex numbers. The imaginary component of χ iscalled its “AC magnetic susceptibility.” A classical interpretation ofthe EPR signal is that the applied magnetic field induces a change inthe AC magnetic susceptibility. Knowledge of ϵ, σ, and μ can be used toidentify fluid components; for example, ϵ is about 80 in water, 2-5 inoil, and 1 in a gas. σ will be virtually zero in hydrocarbons, butnonzero if there is a mix of salty brine water along with the oil. μwill typically be close to 1, but magnetic particles (e.g., from thewall of iron tubulars or from some minerals) can increase μ.

As noted, for example, by U.S. Pat. No. 4,888,554 to Hyde et al.,entitled “Electron Paramagnetic Resonance (EPR) Spectrometer” and issuedDec. 19, 1989 (hereinafter “Hyde '554”), part of the reflected signalwhich is in phase with the RF magnetic field gives the absorptionspectrum, whereas that part which is out of phase gives a dispersionspectrum. The absorption spectrum typically has the more usefulinformation for paramagnetic analysis. Hyde '554 also discloses that acircuit can be added to automatically lock the RF frequency to thecavity resonance frequency.

The incoming RF signal will typically be transported to the resonatorvia a coax cable that has a certain characteristic impedance (e.g.,50Ω). The resonator itself has a different impedance, which will be somecomplex number representing a mostly inductive load. That impedancedepends on the structural design of the resonator, as well as thecontents of the fluid therein. An exemplary EPR design will include animpedance matching circuit that can vary one or more parameters so thatthe combination of the impedance matching circuit and the resonator willbe a load impedance adjusted to match that of the impedance looking backinto the transmit path for a particular frequency or range offrequencies. This matching will minimize reflections in the circuit pathfrom the resonator. In a typical embodiment, the impedance matchingcircuit will vary the capacitance using one or more components, such asa varactor, whose capacitance can be set by varying an externalparameter (e.g., an applied DC voltage). Capacitance is but one aspectof impedance, so it is fair to describe the impedance matching circuitas one which is varying its impedance. The impedance matching process isthus an impedance sweep to match the impedance of the incoming coax tothe impedance of the combination of the impedance matching circuit andthe resonator. The value chosen for that impedance will vary accordingto the electromagnetic properties of the fluid in the resonator cavity.

Until recently, EPR spectrometers comprised components that wereexpensive and large in both weight and physical dimensions. Because ofthis high cost ($500 k), large weight (100 kg), and large size (1 m³),EPR spectrometers were unsuitable for field use in the oil industry,such as for application inside wellbores, at wellheads, or alongpipelines.

Smaller, more portable devices have been introduced in the last fewyears, and these can now provide wellsite solutions by taking samples offluids from the wellbore. The evaluation is performed by inserting thefluid sample into a measurement cavity within the portable EPR device.Spectroscopic information for that fluid is then available to theoperator, providing answers within minutes of taking the sample, withoutthe historical requirement to ship the sample to a chemical laboratoryfor analysis offsite.

Although not specifically targeted to the oilfield, patents disclosingsuch smaller devices include U.S. Pat. No. 8,212,536 to White et al.,entitled “Method and Apparatus for In-situ Measurement of Soot byElectron Spin Resonance (ESR) Spectrometry” and issued Jul. 3, 2012;U.S. Pat. No. 8,829,904 to White et al., entitled “Method of andApparatus for In-situ Measurement of Degradation of Automotive Fluidsand the Like by Micro-electron Spin Resonance (ESR) Spectrometry” andissued Sep. 9, 2014; U.S. Pat. No. 7,868,616 to White et al., entitled“Method of and Apparatus for In-situ Measurement of Changes in FluidComposition by Electron Spin Resonance (ESR) Spectrometry” and issuedJan. 11, 2011; and U.S. Pat. No. 5,233,303 to Bales et al., entitled“Portable Dedicated Electron Spin Resonance Spectrometer” and issuedAug. 3, 1993 (hereinafter “Bales '303”). The entire contents of thesefour patents are herein incorporated by reference.

Subsequently, it has been determined that the physical characteristicsof the spectrometer were not the only hurdles to installation for wellor pipeline applications. By taking measurements every few hours on aportable device at the wellsite, it is now known that the EPR propertiesof an oilfield fluid can change dramatically, particularly during achemical treatment or clean-up of the well. It has also been determinedthat exposing a well fluid to oxygen can change the fluid's EPRresponse. EPR responses are further known to change based on the fluidtemperature and pressure. Given the EPR response at wellheadtemperatures and pressures and a separate pressure, volume, andtemperature (PVT) analysis of the fluid, it may be possible to estimateEPR responses upstream (e.g., deeper in the well). However, themeasurement cavity in the portable EPR devices is not in pressurecommunication with the wellhead or surface facilities. Instead, thosedevices operate by taking a sample of fluid from the wellhead (or apipeline exit), transferring that sample to the measurement device, andthen inserting the sample into the device cavity. Such fluid samples aretherefore not at the same pressure, temperature, and other conditions asthese samples would be downhole, in the wellhead, or in the surfaceproduction pipelines. Also, those samples are typically exposed to theatmosphere as part of the transition from wellhead or pipelineinfrastructure to the measurement device. Furthermore, there is still adelay between taking the fluid samples and performing EPR measurementsthereon; thus, the measurement of such samples is not in real-time asthe fluid is flowing in the tubular. This has created the need for anEPR device that could be integrated into oilfield apparatus and thatcould make continuous EPR measurements of flowing fluid under actualconditions, without exposing that fluid to the air and without bringingthe typically multiphase well fluid to atmospheric temperature andpressure.

It is known that in multiphase flow, the fluid may traverse in differentflow regimes (e.g., bubble flow, slug flow, and emulsion flow for twoliquids and bubble flow, dispersed bubble flow, plug flow, slug flow,froth flow, mist flow, churn flow, and annular flow for gas-liquidcombinations). It is also known that for some of these flow regimes,turbulizers can be included in the tubular to make downstreamcross-sections of the pipe more representative of the average flow(e.g., for sampling). For slug flow, however, turbulizers are lessuseful: the first fluid will not become blended with the second. Rather,the two fluids will stay as separate components travelling along thewellbore. Such a scenario is not uncommon for applications of enhancedoil recovery when the wellhead may see many feet of injected water,followed by a few feet of oil, and then many more feet of water.

In the case of wellbore cleanouts for asphaltene or scale removal, largevolumes of solvents, surfactants, or dispersants may be pumped into awellbore and then retrieved back to surface when the well is put backonline. In this scenario, a first fluid will be flowing for many minutesor hours before a more representative sample of reservoir fluid returnsto the surface.

As noted above, the electrical properties of a resonator cavity willchange dependent upon the fluid therein, so for a cavity continuallybeing refreshed with fluid from a flowing wellbore, these parameters canchange quickly as different fluids reach the wellhead. This leads to thedesired capability of quickly changing the frequency of the applied RFmagnetic field to keep the cavity at or near resonance. Similarly, itmay be desirable to quickly update any impedance match to the cavity.

None of the cited patent references above anticipate real-timemeasurements of a flowing fluid with rapidly changing fluid properties,nor do these references anticipate maintaining the fluid in its originalstate of temperature and pressure and with no opportunity for fluidspectral changes as a result of exposure to oxygen. Accordingly, thesereferences and the teachings therein are not applicable to many oilfieldapplications.

SUMMARY

Certain aspects of the present disclosure generally relate to usingelectron paramagnetic resonance (EPR) to analyze a flow system.

Certain aspects of the present disclosure provide a method of performingEPR spectroscopy on a fluid from a flowing well. The method generallyincludes, for a first EPR iteration, performing a first frequency sweepof discrete electromagnetic frequencies on a cavity containing thefluid; determining first parameter values of reflected signals from thefirst frequency sweep; selecting a first discrete frequencycorresponding to one of the first parameter values that is less than athreshold value; activating a first electromagnetic field in the fluidat the first discrete frequency; and while the first electromagneticfield is activated, performing a first DC magnetic field sweep togenerate a first EPR spectrum.

Certain aspects of the present disclosure provide an EPR spectrometerfor performing EPR spectroscopy on a fluid from a flowing well. The EPRspectrometer generally includes a tube having a cavity capable ofreceiving the fluid; a magnetic field generator configured to generate aDC magnetic field in the fluid during operation of the EPR spectrometer;transmit circuitry configured to generate a radio frequency (RF) signal;a resonator coupled to the transmit circuitry and configured to convertthe RF signal into an RF magnetic field in the fluid during theoperation of the EPR spectrometer; receive circuitry configured toreceive and process reflected signals from the fluid via the resonator;and at least one processor coupled to the magnetic field generator, thetransmit circuitry, and the receive circuitry. For a first EPRiteration, the at least one processor is generally configured to controlthe transmit circuitry to perform a first frequency sweep of discreteelectromagnetic frequencies on the cavity containing the fluid; todetermine first parameter values of reflected signals from the firstfrequency sweep; to select a first discrete frequency corresponding toone of the first parameter values that is less than a threshold value;to control the transmit circuitry to activate a first electromagneticfield in the fluid at the first discrete frequency; and to control themagnetic field generator, while the first electromagnetic field isactivated, to perform a first DC magnetic field sweep to generate afirst EPR spectrum.

Certain aspects of the present disclosure provide a non-transitorycomputer-readable medium storing instructions that, when executed on aprocessor, perform operations for performing EPR spectroscopy on a fluidfrom a flowing well. For a first EPR iteration, the operations generallyinclude performing a first frequency sweep of discrete electromagneticfrequencies on a cavity containing the fluid; determining firstparameter values of reflected signals from the first frequency sweep;selecting a first discrete frequency corresponding to one of the firstparameter values that is less than a threshold value; activating a firstelectromagnetic field in the fluid at the first discrete frequency; andwhile the first electromagnetic field is activated, performing a firstDC magnetic field sweep to generate a first EPR spectrum.

Certain aspects of the present disclosure provide a method of performingEPR spectroscopy on a fluid from a flowing well. The method generallyincludes activating a magnetic field generator to generate a DC magneticfield at a first magnetic flux density in a cavity containing the fluid;while the DC magnetic field is activated at the first magnetic fluxdensity, performing at least one of: a frequency sweep to determine afrequency for generating a radio frequency (RF) magnetic field in thefluid; or an impedance sweep to set an impedance of an impedancematching circuit associated with the generation of the RF magneticfield; and sweeping the activated DC magnetic field from the firstmagnetic flux density to a second magnetic flux density using at leastone of the determined frequency or the impedance to generate an EPRspectrum.

Certain aspects of the present disclosure provide an EPR spectrometerfor performing EPR spectroscopy on a fluid from a flowing well. The EPRspectrometer generally includes a tube capable of receiving the fluid; amagnetic field generator configured to generate a DC magnetic field inthe fluid during operation of the EPR spectrometer; transmit circuitryconfigured to generate a radio frequency (RF) signal; a resonatorcoupled to the transmit circuitry and configured to convert the RFsignal into an RF magnetic field in the fluid during the operation ofthe EPR spectrometer; impedance matching circuitry coupled between thetransmit circuitry and the resonator; receive circuitry configured toreceive and process reflected signals from the fluid via the resonator;and at least one processor coupled to the magnetic field generator, thetransmit circuitry, and the receive circuitry. The at least oneprocessor is generally configured to activate the magnetic fieldgenerator to generate the DC magnetic field at a first magnetic fluxdensity; to control, while the DC magnetic field is activated at thefirst magnetic flux density, at least one of: the transmit circuitry toperform a frequency sweep to determine a frequency for generating the RFmagnetic field in the fluid; or the impedance matching circuitry toperform an impedance sweep to set an impedance of the impedance matchingcircuitry associated with the generation of the RF magnetic field; andto control the magnetic field generator to sweep the activated DCmagnetic field from the first magnetic flux density to a second magneticflux density using at least one of the determined frequency or theimpedance to generate an EPR spectrum.

Certain aspects of the present disclosure provide a non-transitorycomputer-readable medium storing instructions that, when executed on aprocessor, perform operations for performing EPR spectroscopy on a fluidfrom a flowing well. The operations generally include activating amagnetic field generator to generate a DC magnetic field at a firstmagnetic flux density; while the DC magnetic field is activated at thefirst magnetic flux density, performing at least one of: a frequencysweep to determine a frequency for generating a radio frequency (RF)magnetic field in the fluid; or an impedance sweep to set an impedanceof an impedance matching circuit associated with the generation of theRF magnetic field; and sweeping the activated DC magnetic field from thefirst magnetic flux density to a second magnetic flux density using atleast one of the determined frequency or the impedance to generate anEPR spectrum.

The foregoing has outlined rather broadly various features of thepresent disclosure in order that the detailed description that followsmay be better understood. Additional features and advantages of thedisclosure will be described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description,briefly summarized above, may be had by reference to aspects, some ofwhich are illustrated in the appended drawings. It is to be noted,however, that the appended drawings illustrate only certain typicalaspects of this disclosure and are therefore not to be consideredlimiting of its scope, for the description may admit to other equallyeffective aspects.

FIG. 1 is a block diagram of an electron paramagnetic resonance (EPR)spectrometer, in accordance with certain aspects of the presentdisclosure.

FIGS. 2A and 2B conceptually illustrate magnetic and electricalcomponents, respectively, of a microwave field in an example EPRresonator, in accordance with certain aspects of the present disclosure.

FIG. 3 is a cross-section of an example resonator containing a chamber,in accordance with certain aspects of the present disclosure.

FIG. 4 is a cut-away view of an example loop-gap resonator with achamber in the cavity, in accordance with certain aspects of the presentdisclosure.

FIG. 5 illustrates an example electromagnet configuration for EPR, inaccordance with certain aspects of the present disclosure.

FIG. 6 is an illustration of a coil on a flexible circuit that cancreate a modulated magnetic field in a resonator cavity, in accordancewith certain aspects of the present disclosure.

FIG. 7 is a block diagram of an example EPR system at a wellsite, inaccordance with certain aspects of the present disclosure.

FIG. 8 is a block diagram of an example transceiver for an EPR system,in accordance with certain aspects of the present disclosure.

FIG. 9 is a block diagram of an example EPR system illustrating feedbackloops for impedance and frequency adjustment, in accordance with certainaspects of the present disclosure.

FIG. 10 is an example graph illustrating reflected signal parametersfrom a sweep of discrete frequencies and identification of a resonancepoint, in accordance with certain aspects of the present disclosure.

FIG. 11 is a flow diagram of example operations for determining bothdielectric permittivity information and an impedance match, inaccordance with certain aspects of the present disclosure.

FIG. 12 is a flow diagram of example operations for deriving an EPRspectrum using low-frequency magnet modulation, in accordance withcertain aspects of the present disclosure.

FIG. 13A is a graph of an example EPR response over a relatively largermagnetic sweep range, in accordance with certain aspects of the presentdisclosure.

FIG. 13B is a graph of an example EPR response over a relativelynarrower magnetic sweep range associated with asphaltene in crude oil,in accordance with certain aspects of the present disclosure.

FIG. 14 is a flow diagram of example operations for deriving a local EPRspectrum by performing impedance and/or frequency sweeps with the DCmagnet activated, in accordance with certain aspects of the presentdisclosure

FIG. 15 illustrates an EPR waveform after baseline removal to give aclean peak-to-peak voltage (Vpp) spectrum for a desired component, inaccordance with certain aspects of the present disclosure.

FIG. 16 is an example plot of EPR curves versus time for an EPRspectrometer, in accordance with certain aspects of the presentdisclosure.

FIGS. 17 and 18 are flow diagrams of example operations for performingEPR spectroscopy on a fluid, in accordance with certain aspects of thepresent disclosure.

DETAILED DESCRIPTION

Certain aspects of the present disclosure provide methods and apparatusfor performing electron paramagnetic resonance (EPR) spectroscopy on afluid.

Refer now to the drawings wherein depicted elements are not necessarilyshown to scale and wherein like or similar elements are designated bythe same reference numeral through the several views.

Referring to the drawings in general, it will be understood that theillustrations are for the purpose of describing particularimplementations of the disclosure and are not intended to be limitingthereto. While most of the terms used herein will be recognizable tothose of ordinary skill in the art, it should be understood that whennot explicitly defined, terms should be interpreted as adopting ameaning presently accepted by those of ordinary skill in the art at thetime of filing the present disclosure.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory only,and are not restrictive of the invention, as claimed. In thisapplication, the use of the singular includes the plural, the word “a”or “an” means “at least one,” and the use of “or” means “and/or,” unlessspecifically stated otherwise. Furthermore, the use of the term“including,” as well as other forms, such as “includes” and “included,”is not limiting. Also, terms such as “element” or “component” encompassboth elements or components comprising one unit and elements orcomponents that comprise more than one unit, unless specifically statedotherwise.

Electron paramagnetic resonance (EPR) is a known technique to deriveparamagnetic characteristics of materials by exposing these materials toa combination of magnetic and electromagnetic fields that inducesresonance of unpaired electrons within the materials. Discussion of EPRprinciples and techniques can be found in J. A. Weil and J. R. Bolton,Electron Paramagnetic Resonance: Elementary Theory and PracticalApplications, 2^(nd) Ed., Hoboken, N.J.: John Wiley & Sons, 2007;Gilbert et al., Electron Paramagnetic Resonance, Volume 20, The RoyalSociety of Chemistry, Cambridge UK 2007; A. Schweiger and G. Jeschke,Principles of Pulse Electron Paramagnetic Resonance, Oxford UniversityPress, 2001; and G. R. Eaton, S. S. Eaton, D. P. Barr, and R. T. Weber,Quantitative EPR, Vienna: Springer, 2010.

According to certain aspects of the present disclosure, the stepsgenerally involved in EPR measurement may include first estimating theresonant frequency of an enclosure cavity containing the wellbore fluidsample, exciting a uniform magnetic field, B₁, at or near the resonantfrequency by coupling against an incoming radio frequency (RF) field,and then applying a strong static magnetic field, B₀, that is orientedlargely perpendicular to the first magnetic field. The strength of thatDC magnetic field may then be swept to create the desired EPR spectrumbased on reflection. For a given paramagnetic species, the range of themagnetic sweep involved scales with the excitation frequency.

Application of EPR to the oilfield industry has been disclosed in U.S.Patent Publication No. 2016/0223478 to Babakhani et al., entitled “EPRSystems for Flow A Assurance and Logging” and filed Sep. 25, 2014, theentire contents of which are incorporated by reference herein. Babakhaniet al. point out that until recently, EPR spectrometers comprisedcomponents that were expensive, heavy, and large. More portable deviceshave been disclosed in the past few years. In particular, U.S. Pat. No.9,689,954 to Yang et al., entitled “Integrated Electron Spin ResonanceSpectrometer” and issued Jun. 27, 2017, discloses a technique tosignificantly reduce the size of the spectrometer by incorporating themicrowave circuitry onto an integrated circuit. Such size spectrometerreduction may permit using EPR sensors in applications that werepreviously unachievable due to size and portability constraints. Forexample, at the wellsite, the resulting sensor can detect contributionsfrom heavy oil, hydrocarbons, asphaltenes, vanadium, resins, drillingfluid, mud, wax deposits, and the like.

Although such portable EPR devices have been developed, it has beendetermined that the physical characteristics of the spectrometer werenot the only hurdles to installation for well or pipeline applications.By taking measurements every few hours on a portable EPR device at thewellsite, it is now known that the EPR properties of an oilfield fluidcan change dramatically, particularly during a chemical treatment orclean-up of the well. It has also been determined that exposing a wellfluid to oxygen (e.g., in the air) can change the fluid's EPR response.EPR responses are also known to change based on the fluid temperatureand pressure.

This has created the need for a device that can be integrated intooilfield apparatus and that can make continuous EPR measurements of amultiphase (flowing) fluid, without exposing that fluid to the air, andwithout bringing the well fluid to atmospheric temperature and pressure.Certain aspects of the present disclosure provide techniques andapparatus for such an EPR device.

FIG. 1 is a block diagram of an example EPR spectrometer 100, inaccordance with certain aspects of the present disclosure. The EPRspectrometer 100 may generally use building blocks similar to those of atraditional EPR spectrometer. For example, the EPR spectrometer 100 mayinclude one or more magnets 101, a resonator 103, and a transceiver 107,which includes both transmit (TX) circuitry 108 and receive (RX)circuitry 109 (also referred to as a transmitter and a receiver,respectively).

For certain aspects, the transceiver 107 may be a microwave transceiver,operating at frequencies between 300 MHz and 300 GHz, for example. TheTX circuitry 108 may include a frequency synthesizer 110 and a poweramplifier 111 coupled between the output of the frequency synthesizer110 and a (e.g., port 1) of the circulator 106. The TX circuitry 108 iscoupled to the resonator 103 via a circulator 106, so that the energy ofthe source transmission does not overwhelm the sensitive circuits of theRX circuitry 109. The output of the circulator 106 passes to theresonator 103, which creates a radio frequency (RF) electromagneticfield 104 (B₁ field) whose magnetic component is largely perpendicularto that of the static DC magnetic field 102 (B₀ field or Zeeman field).

A magnetic field generator provides the DC magnetic field 102 utilizingmagnets 101, coils, or the like. The resonator 103 and sample chambertherein are placed inside the magnets 101 and/or coils that generate theDC magnetic field B₀. The sample chamber is designed to allow fluids toflow therethrough. The fluid flow might be that of a full tubular inwellsite equipment or a sidestream to which a subset of the main flowhas been directed. In a downhole apparatus, the fluid flow might be thatcoming from a specific interval of the reservoir, such as directed by adownhole control valve or similar device. The presence of the Zeemanfield introduces an energy difference ΔE between the two spin states ofan unpaired electron: parallel and anti-parallel to B₀, with ΔE beingproportional to B₀. At its resonant frequency, the resonator 103produces the RF magnetic field B₁. Using the notation h for the Planckconstant, then at that RF frequency (f) where hf equals ΔE (i.e., theLarmor frequency), spin transitions between the two up and down spinstates occur, resulting in absorption of RF energy in the sample. In areflection-type resonator, this results in a change in the level ofreflected power from the resonator. This reflected power from theresonator is coupled to the receiver via the circulator 106 (at port 3).For certain aspects, the receiver may include a low noise amplifier(LNA) 112, a mixer 113 coupled to the output of the LNA 112 and theoutput of the frequency synthesizer 110, and an amplifier 114 coupled tothe output of the mixer 113.

As noted by International Patent Application Publication No. 2016187300to Babakhani et al., entitled “Electron Paramagnetic Resonance (EPR)Systems with Active Cancellation” and filed May 18, 2016, the circulatormight not provide complete isolation between the TX and RX circuitry, inwhich case an active cancellation component may be added to the EPRspectrometer, as described therein. The entire contents of WO 2016187300are herein incorporated by reference. For example, EPR can also be usedas a tracer monitoring to characterize or measure the concentration ofmagnetic or paramagnetic nano-particle materials in harsh hightemperature high pressure well environments. A non-limiting exampleincludes injecting nanoparticles with different EPR signatures indifferent injection wells. Then using an EPR sensor in a production well(e.g. wellhead) to detect the concentration of the injectednanoparticles in the production well. Nonlimiting applications for suchtechniques include using such measurements to estimate the connectivityof different injection wells to a production well, well mappingincluding the mapping of the well pathway and fractures, or the like.

The active cancellation system and EPR sensors discussed herein may havebroad applicability to various applications involving identifying andlocating certain types of materials. In some embodiments, the activecancellation system and EPR sensor can be utilize for a method ofdetecting EPR spectrum of transitional metals, asphaltenes, vanadium,Fe2+, Mn2+, organic materials in crude oil, Kerogen, naturally occurringfree radicals, magnetic nano-particles, and scale. The EPR response ofKerogen in rock, rack samples, shale, etc. can be used for the purposeof characterization, type of and amount of Kerogen, includingdetermining maturity of hydrocarbon within the rock. In thisapplication, we can filter and sample rock (e.g. bit cuttings) at thesurface during drilling operations, as a nonlimiting example during LWD(Logging While Drilling) & MWD (Measurement While Drilling) operations,in a dedicated tool measuring rock formation. This may occur duringdrilling at or close behind drill bit or in a laboratory setting.

EPR can be used to characterize asphaltenes in the flow of fluidsretrieve from the well or in the rock formation. It can also be used tocharacterize asphaltenes in the sample rock (e.g. bit cuttings) at thesurface during drilling operations; during LWD & MWD operations indedicate tool measuring rock formation during drilling at or closebehind drill bit; or in a laboratory setting.

EPR can be used, but not limited to, as a dedicated tool for LWD & MDWoperations. These measurements can be done in dedicated wireline tool;after drilling as open hole reservoir characterization tool; or later inthe life of the well as reservoir characterization and optimizationtool. Non-limiting examples include asphaltene profiling throughreservoir, kerogen profiling throughout formations, or the like.Applications include characterizing kerogen, asphaltene, or the like intight shale source rock, conventional, or unconventional reservoirs.

EPR can also be used as a tracer monitoring to characterize or measurethe concentration of magnetic or paramagnetic nano-particle materials inharsh high temperature high pressure well environments. A non-limitingexample includes injecting nanoparticles with different EPR signaturesin different injection wells. Then using an EPR sensor in a productionwell (e.g. wellhead) to detect the concentration of the injectednanoparticles in the production well. Nonlimiting applications for suchtechniques include using such measurements to estimate the connectivityof different injection wells to a production well, well mappingincluding the mapping of the well pathway and fractures, or the like.

EPR can also be used in downstream industry to improve the efficiencyand use of chemicals in manufacturing processes. Non-limiting examplesinclude measuring asphaltenes concentration in oil refining processes,measuring oxygen properties in process of making beer, measuring thelevel of free-radicals in food processing, measuring the quality ofengine oil in motors or vehicles, or the like.

EPR can also be used to measure the type and severity of corrosion bymeasuring the concentration of transition metals or metal ions such asFe2+, Mn2+, or Fe304, or Fe203 in a flow (e.g. in pipelines).

As a nonlimiting example, the EPR spectrum can be used to monitorcorrosion processes (e.g. Fe2+, Mn2+). The EPR spectrum can be used tomonitor asphaltenes aggregation and concentration in flow-assurance. Itcan also be used to determine flow composition and flow rate bymeasuring the asphaltenes concentration and velocity. Magneticnanoparticles can be used as tracers in exploration and EPR can detectthem. For flow-assurance, it is possible to build a closed-loop systemwhere EPR sensor detects a problem (such as asphaltenesaggregation/deposition) and chemical injection is used to mitigate theproblem (the claim on closed-loop system). FIG. 22 shows EPR recordervoltage v. current for several trials.

The resonator 103 may be excited with continuous wave or pulsedexcitation. In one aspect, the EPR sensor is a sensor that operates at 1GHz or higher. In other aspects, the EPR sensor may operate at lowerfrequencies. For certain aspects, the EPR sensor may operate in therange of 3-5 GHz.

The design and construction of the resonator and its resonant cavity areimportant. The resonant cavity may most likely be designed such that theamplitude of the electrical component will be typically large over someregion of the resonator, while the magnetic field is large over adifferent region. For example, Eaton (op. cit.) gives this comparison ofmagnetic and electric fields in the typical cavity of some commercialEPR systems, which are represented in FIGS. 2A and 2B. FIG. 2Aillustrates a sample cavity 201 in an example resonator 202 withmicrowave magnetic field lines 203 depicted. FIG. 2B illustrates thesame resonator 202 with microwave electric field lines 204 portrayed.

The sample (e.g., a flowing fluid from hydrocarbon recovery operations)should ideally be inserted into the region with large magnetic field, asopposed to the region with large electric field. This is particularlyimportant in the case of oilfield flow sensing, because the fluid willtypically contain at least some component of brine, which iselectrically conductive. Oilfield fluid might also be corrosive (e.g.,if acids are pumped into the well) and/or erosive (e.g., if there is asignificant quantity of solids produced from the well or scraps ofmetal, etc., from tubular walls). Conventionally, the fluid wastypically sampled at atmospheric pressure and temperature, but certainaspects of the present disclosure can sample the fluid under the actualhigh temperature and high pressure conditions experienced downhole, atthe wellhead, or within the production pipeline system. In particular,for certain aspects, the fluid may remain in pressure communication withthe wellbore so that the sample passing through the resonator isrepresentative of the fluid passing through the wellbore.

With regard to the above resonant cavity design, the frequency settingon commercial EPR devices using this structure typically employs amechanical tuning device. Certain aspects of the present disclosureoperate autonomously, and thus, the tuning of the EPR device may beunder processor control. Furthermore, a tuning screw would not bemechanically sound in the presence of the routine vibration seen onobjects attached to wellheads and pipelines. For certain aspects of thepresent disclosure, there are no moving components to accomplishinductive coupling to the cavity or to accomplish resonance ornear-resonance of the RF magnetic field.

These and other design considerations may lead to the example resonator300 of FIG. 3, shown in cross-section, in accordance with certainaspects of the present disclosure. In the resonator 300, the sample(e.g., the oilfield fluid) may be contained in a tube 301, which may bea pressure-bearing chamber for withstanding the high temperatures andhigh pressures experienced downhole. The tube 301 may be made frompolyether ether ketone (PEEK), with a possible manufacturer beingVictrex PLC of Lancashire, United Kingdom(https://www.victrex.com/en/victrex-peek). PEEK is non-conducting,non-magnetic, and relatively transparent to electromagnetic (EM) wavesin the GHz regime, so that PEEK can be incorporated within the structureof a loop-gap resonator. For certain aspects, the tube 301 may be acylindrical container. The walls of the tube 301 may be surrounded by ahousing 302. The housing 302 may also be cylindrical for certainaspects.

The cross-section shown in FIG. 3 is of the “gap” section of theresonator 300. In the gap section, the resonator 300 may have multiplegaps 303 in the housing 302 to concentrate the electric field in thesegaps. Suitable performance may be obtained with 2 to 5 gaps, forexample, although more or less than this range of gaps may be used. Forexample, S. Petryakov et al., “Single Loop—MultiGap Resonator for WholeBody EPR Imaging of Mice at 1.2 GHz,” Journal of Magnetic Resonance,v188(1), pp. 68-73 (September 2007), describes the use of 16 gaps. Theresonator 300 in FIG. 3 is depicted with three gaps 303. The housing 302may be composed of any suitable electrically conductive material, suchas metal. For certain aspects, copper (Cu) is used for the metal wallsof the housing 302. To maximize sample area within the resonator 300,the metal walls of the housing 302 may be tightly bound onto the tube301, minimizing the interface 304 therebetween.

To excite this resonator structure having a chamber in the cavity, a(circular) loop 402 (a coil) may be added to the flow system 400 abovethe gap section of FIG. 3, as shown in the cut-away view of FIG. 4, toform a loop-gap resonator. The tube 301 may be formed as a cylindricalchamber, with the loop 402 perpendicular to the longitudinal axis 404 ofthe cylinder. Although the loop 402 is illustrated as being circular inFIG. 4, the loop may have other suitable shapes, such as elliptical oroval. These shapes may depend on the cross-sectional shape of thehousing 302. The loop-gap resonator of FIG. 4 may be constructed with atight bond between the tube inlet, the housing 302, and the loop 402.This may provide structural integrity in order to survive vibrationcaused by the moving fluid. In one aspect, the height Z is about 1 inch(2.54 cm), and the inner diameter of the tube 301 is about ¼″, which isa significantly higher diameter than is common in the EPR industry.

It is noted that other resonator designs are well known in the industry.U.S. Patent Application Publication No. 2015/0185299 to Rinard et al.,entitled “Crossed-Loop Resonators” and filed Jan. 12, 2015, for example,discloses a crossed-loop resonator (CLR) that could also be applicable.The crossed-loop resonator uses two orthogonal lumped-elementresonators—one to excite the spins and one to detect the electronparamagnetic resonance—to isolate the signal from the microwave source.Therefore, an EPR system implemented with a CLR may not include acirculator. As noted by Rinard, the high isolation provided by the CLRreduces the energy stored in the resonator that detects the signal,thereby reducing the intensity of the resonator ring down after thepulse, which decreases the instrument dead time. Another alternativedesign using surface-coil type resonators is given by H. Yokoyama and T.Yoshimura, “Combining a magnetic field modulation coil with asurface-coil-type EPR resonator,” Applied Magnetic Resonance, Vol. 35,Issue 1, p. 127-135 (November 2008). A bimodal resonator has beendescribed in Sundramoorthy et al., “Orthogonal Resonators for Pulse InVivo Electron Paramagnetic Imaging at 250 MHz,” Journal MagneticResonance, Vol. 240, pp. 45-51 (March 2014). The bimodal resonatorachieves a 19 mm internal diameter with improved B₁ homogeneity comparedto a loop-gap resonator of the same size and volume. The entire contentsof these three documents are herein incorporated by reference.

As described above, static magnetic excitation for EPR can be performedwith an electromagnet. FIG. 5 illustrates an example electromagnet 500,which may be implemented in certain aspects of the present disclosure.FIG. 5 also includes two-dimensional (2-D) computer-aided design (CAD)drawings 520 and a three-dimensional (3-D) CAD rendering 540 for theelectromagnet 500. One electromagnet 500 may be positioned on eitherside of the sample tube (e.g., tube 301). Each electromagnet 500 mayhave 650-750 turns of 18 AWG magnet wire, for example. The magnet wiremay be wound to form a coil 502 with high temperature epoxy and a 0.005″fiber glass tape used for insulation between turns. The entire assemblyis rigidly housed in a non-magnetic housing 504. In an electromagnet,increasing the voltage on the coil increases the strength of the magnet(i.e., the magnetic flux density), which thereby enables a magneticsweep.

Other magnetic configurations may be appropriate in some circumstances.As noted by Bales '303, for example, a further permanent magnet can beadded to reduce the range specified for the magnetic sweep. Such apermanent magnet helps decrease the size of the electromagnet specifiedand so helps reduce the overall size and weight of an EPR system. Themagnetic excitation of the electron spins may be accomplished with thecombination of a static DC magnetic field generated by an electromagnetwhose magnitude can be swept and an additional coil used to add amodulation frequency (e.g., a lower frequency modulation, such as in theaudio frequency range) to that magnetic field.

Rather than determining the response to a particular magnetizationdirectly, the DC magnet may be modulated with an additional small, lowfrequency (e.g., 100 kHz) coil of an electromagnet that creates alargely uniform magnetic field pointed in the same direction as the DCmagnetic field B₀. For certain aspects, the coil may be created byprinting a circuit (e.g., metal traces) onto a multi-layer flexibleprinted circuit board (PCB) 600 as shown in FIG. 6 and illustrated inthe conceptual diagram of a flow system 620. In the flow system 620, theflexible PCB 600 may be wrapped over the exterior of the (loop-gap)resonator 622 for certain aspects. The flexible circuit can be printedso that when folded in place, the flexible circuit creates two loops ofa Helmholtz coil. Alternatively, as in the example of FIG. 6, theflexible PCB 600 can be folded to create two line sources, one on eitherside of the cavity. The magnetic field produced by the flexible PCB 600is largely uniform over the fluid sample and in the same direction asthat produced by the large electromagnets.

The design of the EPR system according to certain aspects of the presentdisclosure allows the sensitive electronics to be located away from thefluid sample to be probed and provides for significantly more efficientsystems, such as co-placement of high-frequency components onto a singlechip. A resonator (e.g., a loop-gap resonator) may appear as aninductive load on a microwave feed line, so the resonator may mostlikely be impedance matched with a coupling component, such as avaractor, similar to that described by Bales '303.

It is worth noting that the spectroscope need not operate at exactly theresonance frequency, but the closer the operating frequency is toresonance, then the larger the RF magnetic field induced and so thelarger the EPR signal. To avoid dramatic dependence on getting an exactresonant frequency, the EPR spectroscope may have a cavity with aquality factor (Q) that is modestly high (e.g., >50), but not extremelyhigh (e.g., >1000).

FIG. 7 is a block diagram of an example EPR system 700, in accordancewith certain aspects of the present disclosure. As shown, the EPR system700 comprises five modules: a high power programmable current source702, a power module 704, a controller module 706, a transceiver module708, and a resonator assembly 710. The high power programmable currentsource 702 may be implemented by a power supply with, for example, again of 5 A/V capable of 10 A with a 100 m H load. For certain aspects,an appropriate level of accuracy is 0.1% (±0.01 A). The output of thisprogrammable current source 702 feeds a magnet 711 in the resonatorassembly 710 to control the magnetic field. The controller module 706may be capable of outputting a control voltage (e.g., ranging from 0 Vto 2 V) to control the programmable current source 702. The power module704 is a system capable of transforming mains electricity (e.g., 120 VACat 60 Hz) to one or more DC voltages (e.g., 12 VDC, 5 VDC, and/or 5.5VDC) for use in the EPR system 700. The transceiver module 708 is an EPRfrequency board, capable of generating an RF signal for a resonator inthe resonator assembly 710. Two board options may be considered for thetransceiver module: an integrated circuit (IC) transceiver board and adiscrete component transceiver board. For example, the discretecomponent transceiver board may use a 12 VDC power supply voltage outputby the power module 704. Alternatively, the IC transceiver board may usea 5 VDC power supply voltage, which may be buffered through thecontroller module 706.

The EPR system 700 may also include a computer 712 or any of variousother devices with a suitable processing system (e.g., a tablet, asmartphone, and the like). The computer 712 is capable of sendingcommands to and receiving data from the controller module 706 (e.g., viaa USB/UART bridge 714).

As shown in FIG. 7, the EPR system 700 may remain in continuous fluidcommunication with equipment at a wellsite, such as a wellhead 716disposed at the surface and/or production tubing 718 disposed in awellbore. The production tubing 718 may be one of multiple tubulars inthe wellbore. It is not uncommon, for example, that the productiontubing 718 is contained within a number of strings of casing (notshown). The wellhead 716 as drawn figuratively represents the connectionbetween a surface production pipeline 720 and the production tubing 718.As is well known in the industry, wellheads typically have a number ofsample ports thereon, which allows an operator access to the fluidflowing from a reservoir. During production, the flow path from theproduction tubing 718 through the wellhead 716 to the pipeline 720 isgenerally maintained as a pressure barrier to disallow reservoir fluidsfrom polluting the air and ground nearby. Consequentially, the fluidcommunication channels 722, 724 from the wellhead 716 to the resonatorassembly 710 and back should be able to withstand internal fluidpressure. The connections of the channels 722, 724 to the wellhead 716may be permanently welded or may be hose connections that are certifiedfor exposure to oilfield fluids and pressures.

As drawn, the fluid connection for the channels 722, 724 is madedownstream of the wellhead 716 and upstream of the surface pipeline 720,but other configurations may be utilized, which will be clear to thoseskilled in the art. For example, the connections may be located furtherdownstream, such as in the vicinity of a pipeline manifold or at samplepoints along a pipeline as the pipeline transfers fluid from thewellbore to a refinery or vessel. Alternatively, the connections may bebelow the wellhead 716, such as in a scenario where the resonatorassembly 710 is incorporated as an in-well sensor.

The computer 712 may be some significant distance away from thewellhead. In this case, the computer 712 may be in communication withthe wellsite equipment by means of the cloud or other communicationsnetwork. Indeed, in a typical oilfield setting, some components may needto be positioned close to the wellbore, while others may need to belocated relatively far away. RF components, such as the resonator andthe transceiver should be typically spaced within a few feet of eachother, and to keep the channels 722, 724 short, the resonator may mostlikely also be positioned within a few feet of the wellhead. This meansthat these RF components may most likely be enclosed in one or moreexplosion-proof housings to avoid any safety issues, should there beaccidental release of hydrocarbon at the wellhead. The power supplies,audio-frequency devices, etc. can be some distance removed from thewellhead without issue, so these components need not be inexplosion-proof housing(s), but might benefit from being in housings toprovide insulation from the rain, snow, heat, etc.

FIG. 8 is a block diagram of an example transceiver 800 for an EPRsystem, in accordance with certain aspects of the present disclosure.For example, the transceiver 800 may be implemented in the transceivermodule 708 of FIG. 7. The transceiver 800 may include a transmit chainthat includes a frequency synthesizer 802 (e.g., a PLL), an amplifier804 (e.g., a high frequency (HF) low noise amplifier (LNA)), anattenuator 806, and a splitter 808 for generating a radio frequency (RF)signal for activating an RF field in the resonator cavity. To couple theRF signal to the resonator, the transceiver 800 may also include acirculator 810, an impedance matching circuit 812 (comprising, e.g., avaractor), and a connector 814 (e.g., an SMA coaxial connector). Thetransceiver 800 may also include a receive chain that includes a filter816 (e.g., a high pass filter or a bandpass filter), an amplifier 818(e.g., an HF LNA), a coupler 820 (e.g., a 10 dB coupler), a mixer 822, afilter 824 (e.g., a low-pass filter), and an amplifier 826 (e.g., a lowfrequency (LF) LNA). The transceiver 800 may also include a powerdetector 828.

As described above, the purpose of the impedance matching circuit 812 isto make the combined impedance of this circuit and the resonator matchthat of the effective impedance looking back into the transmit path fora particular frequency or range of frequencies. The resonator maytypically appear as a mostly inductive load, so the impedance matchingcircuit 812 may be designed to have a capacitive component. Theimpedance matching circuit 812 may therefore vary the capacitance(s) ofthe capacitive element(s) to effectively adjust the impedance in aneffort to minimize reflections. It will be apparent to those skilled inthe art that this matching technique may be referred to as “an impedancesweep to set the impedance of an impedance matching circuit associatedwith the generation of the RF magnetic field.” The value chosen for thatimpedance (or, more specifically in some cases, the capacitance) willvary according to the electromagnetic properties of the fluid in thecavity.

For one aspect, the following is a list of the key components for thedifferent blocks of the transceiver 800:

-   -   frequency synthesizer 802: HMC837 Evaluation Board        (http://www.analog.com/en/products/rf-microwave/pll-synth/fractional-n-plls/hmc837.html);    -   circulator 810: DITOM D3C4450        (https://www.ditom.com/images/D3C4450.pdf);    -   splitter 808: RF-Lambda RFLT2W2G08G        (http://www.rflambda.com/pdf/medpowercombinersplitter/RFLT2W2G08G.pdf)    -   attenuator 806: Mini-Circuits VAT-3+        (http://www.minicircuits.com/pdfs/VAT-3+.pdf);    -   amplifier 804 or 818 (e.g., a HF LNA): RF-Lambda RLNA01M06GE        (http://www.rflambda.com/pdf/lownoiseamplifier/RLNA01M06GE.pdf);    -   coupler 820: Mini-Circuits ZUDC10-183+        (https://www.minicircuits.com/pdfs/ZUDC10-183+.pdf);    -   power detector 828: RF Bay RPD 5501        (http://rfbayinc.com/products_pdf/product_445.pdf);    -   filter 816 (HP filter: Mini-Circuits VHF-3100+        (https://www.minicircuits.com/pdfs/VHF-3100+.pdf);    -   filter 824 (LP filter): Mini-Circuits SLP-1.9+        (http://www.minicircuits.com/pdfs/SLP-1.9.pdf);    -   amplifier 826 (LF LNA): RF Bay LNA 1800        (http://rfbayinc.com/products_pdf/product_94.pdf);    -   mixer 822: Fairview MW SFM2018        (https://www.fairviewmicrowave.com/images/productPDF/SFM2018.pdf);    -   impedance matching network 812 (e.g., a varactor board): DAC        LTC2615 (http://www.linear.com/product/LTC2615); and    -   connector 814 (SMA):        (https://cinchconnectivity.com/OA_MEDIA/specs/pi-142-0701-801.pdf).

Additional components may be added to the circuit shown in FIG. 8. Forexample, in certain aspects, one or more filters may be added to thecircuit. One non-limiting example of a filter includes a DC blockingfilter (e.g., a high-pass filter or a bandpass filter), which may beconnected in series with the impedance matching network 812 (e.g., withthe variable capacitor). The impedance control signal (e.g., output bythe controller module 706 and connected to the transceiver 800 via aconnector 832) may be an analog control signal or a digital controlsignal (e.g., for a switched capacitor). When this combination is used,extremely fast feedback is possible. Note that the PLL HMC767 has asettling time of 433 μs and the DAC LTC2615 has a settling time of just7 μs. Yet, still faster operation is possible as will be clear to thoseskilled in the art.

With the recent advancement in fast-settling PLL frequency synthesizertechnologies, it is possible to change the frequency of the localoscillator (LO) 834 in the EPR system in a few microseconds. Forexample, Analog Devices ADF4193 is a low phase noise, fast settling PLLfrequency synthesizer available from Analog Devices, Inc. of Norwood,Mass. The ADF4193 provides frequency hopping in 5 μs and phase settlingby 20 μs. The following is a hyperlink to the datasheet for the ADF4193:http://datasheet.octopart.com/ADF4193BCPZ-Analog-Devices-datasheet-10548259.pdf.

Furthermore, with the advancement of digital-to-analog converter (DAC)technologies and drivers for capacitive loads, it is possible to changethe value of the varactor(s) used in the matching network in timescaleson the order of 10 ns. For example, Texas Instruments DAC39J84 is aquad-channel, 16-bit, 2.8 giga-samples per second (Gsps) interpolatingDAC available from Texas Instruments Inc. of Dallas, Tex. The DAC39J84provides sample rates exceeding 1 GHz that can be used to drive avaractor. The following is a hyperlink to the datasheet for theDAC39J84: http://www.ti.com/product/DAC39J84/datasheet/abstract#SLASE1644.

Although the DAC can operate in timescales shorter than 1 ns, thesettling time of the impedance matching network 812 may be fundamentallylimited by the quality factor (Q) of the matching network. For example,for a resonance frequency of 5 GHz and Q of 100, the settling time maybe limited to 200 ps×100=20 ns.

For certain aspects, the EPR system may also measure the magnetic fieldinside the cavity through the addition of a Hall effect sensor, orequivalent, as disclosed, for example, by U.S. Patent ApplicationPublication No. 2015/0185255 to Eaton et al., entitled “Hall Probe, EPRCoil Driver and EPR Rapid Scan Deconvolution” and filed Feb. 5, 2015,the entire contents of which are herein incorporated by reference. Theoutput of the EPR spectrum can be scaled by this measured magnetic fieldvalue to allow estimation of the number of electron spins per unitvolume (“Ng” in the terminology of EPR spectral analysis).

Considering these improvements as part of the EPR system 900, as shownin FIG. 9, the overall feedback process can be done very fast, allowingthe network to adjust in real-time to changes in the flowing fluid. Theflow system 910 has an impedance that is a function of the resonatordesign, the materials used in the resonator, and the properties of thefluid flowing therethrough. Thus, the flow system 910 may changes itsimpedance continuously while the fluid is flowing, especially in thecase of multiphase fluid from hydrocarbon recovery operations.Therefore, to achieve maximum transfer of energy and low powerreflection, the impedance matching network 908 should adjust itsimpedance to make a match between the effective impedance looking backinto the transmit path and the combined impedance of the matchingnetwork 908 and the resonator surrounding the sample chamber in the flowsystem 910.

As noted above, the network 908 is capable of modifying its impedance bymeans of a programmable matching network in series with the transmissionline. Non-limiting examples of the programmable capacitance include avaractor or a switched capacitor. In the case of a varactor, a DCvoltage controls the capacitance of the varactor. A feedback loop readsthe reflected signal via a directional coupler 912. Non-limitingexamples of the directional coupler 912 would be a 20 dB coupler, a 10dB coupler, a 3 dB coupler, etc. A system with a smaller coupling isdesired because smaller coupling results in a smaller loss in the mainEPR signal, which results in a higher EPR power to the input of the EPRreceiver 914. Although the coupler 912 passes the desired EPR signal tothe receiver, the coupler is also used to measure the amount of theundesired reflected signal from the resonator. A smaller reflectedsignal means that more power can be sent to the resonator by thetransmitter 904 without causing saturation of the receiver 914 by thereflected signal. The reflected power coming out of the coupler 912 maybe measured to tune the impedance of the matching network 908 (e.g., thecapacitance of the variable capacitor(s)) and hence maintain desiredmatching.

Since different flow types have different dielectric constants andconductivities, the power of the reflected signal can be periodicallymeasured to determine the impedance of the resonator and its matchingquality. The reflected signal from the coupler 912 may be converted to aDC voltage using a power detector (not shown). This DC voltage then canbe digitized using an analog-to-digital convertor (ADC) (not shown) andfed into the control unit 902. The control unit 902 may be composed ofat least one microprocessor, microcontroller, programmable integratedcircuit (e.g., a field-programmable gate array (FPGA)), orapplication-specific integrated circuit (ASIC). The control unit 902 mayuse an algorithm to generate the control signal 907 to change theimpedance of the matching network 908 (e.g., an actuation signal tochange the value of the tunable capacitor), in an effort to match thisimpedance to the impedance of the resonator in the flow system 910.

It is to be understood that the impedance matching network 908 mayinclude multiple capacitors in parallel and/or in series, some of whichmay be variable, while others may be fixed. These capacitors (fixedand/or variable) may also have switches connected in series or inparallel therewith for effectively enabling and disabling variouscapacitors in an effort to adjust the overall capacitance of theimpedance matching network 908. A person having ordinary skill in theart will understand that the overall impedance matching network 908 mayeffectively have a variable capacitance that can be adjusted (e.g., by acontrol signal 907 output from the control unit 902).

In addition to changing the impedance of the matching network 908, thecontrol unit 902 may adjust the frequency of the transmitter 904 toachieve a substantial value of the reflected signal. As a non-limitingexample, the mechanism for changing the impedance of the matchingnetwork 908 or the frequency of the transmitter 904 may be accomplishedby using one or more digital-to-analog converters (DACs) inside orexternal to the control unit 902. In this case, a DAC may receive adigital signal from the control unit 902, generate an analog signal(e.g., control signal 907), and control the impedance of the matchingnetwork 908 (e.g., the variable capacitance of a varactor). In this caseanother DAC may be used to control the frequency of the transmitter 904by adjusting the tuning voltage of a voltage-controlled oscillator(VCO). Alternatively, the control unit may send a digital control signalto a programmable phase-locked loop (PLL) to set the frequency of thetransmitter 904.

Circuitry can be included to automatically set the transmitter frequencyto the resonant frequency of the resonator cavity. This need not be theoptimal strategy, however. For example, to obtain the most stablefrequency, it can be advantageous to have a frequency synthesizer (e.g.,in the transmitter 904) that operates at discrete intervals, so that thesynthesizer can get to the discrete frequency interval (e.g., 100 kHz)at or closest to the resonant frequency. In this case, the cavity maynot be fully excited to resonance, but one can be confident that therewould be extremely minimal phase noise and jitter in the value of thatfrequency. It has been noted earlier that it is not the resonancecondition which creates an EPR signal; rather, the key is having asufficiently large RF magnetic field perpendicular to the DC magneticfield.

For certain aspects, a circuit could be automatically locked to theclosest discrete frequency step (e.g., 100 kHz) with no additionalinformation being recorded. For other aspects, however, the RF frequencymay first be swept, and the reflected signal may be measured, to yield agraph for the cavity response, similar to the graph 1000 of FIG. 10.

In FIG. 10, the axis 1002 for the reflected signal can represent thereflected total amplitude or the reflected in-phase contribution(voltage or power). The frequency steps may be set by the frequencysynthesizer (e.g., implemented by a phase-locked-loop (PLL) or VCO) inthe EPR system. The condition dB0 represents a preset value known togive an acceptable response of the EPR machine: any value of frequencywith reflection signal less than dB0 will provide enough RF magneticfield strength to create a measurable EPR response. It can be seen thatthe discrete frequency with the lowest reflected signal is not exactlyequal to the resonant frequency. Moreover, when comparing data from onemagnetic sweep to another, if the fluids have not changed, it isadvantageous to be using the same discrete frequency; otherwise, thiscan create an artificial offset between the two EPR responses.

This leads to the following search strategy: sweep over a given range offrequencies, then for those frequencies giving a signal less than dB0,choose that frequency which is equal or closest to the previousfrequency used. This technique has been found to give very stable EPRresponse as fluids flow through the cavity. This technique also avoidsthe possibility of an automated frequency control loop locking into anerroneous value (as has been cited as a problem by Hyde '554, forexample).

There remains a further improvement that can be made. It is known thatcomplex permittivity data can be derived through measurements of theresonance frequency of a cavity (e.g., see “Measurement of ComplexPermittivity and Permeability through a Cavity PerturbationMeasurement,” Master's Thesis in Applied Physics by Tomas Rydholm,Chalmers University of Technology, Sweden, 2015). For example, water(which typically has a dielectric constant near 80) reduces the resonantfrequency of the resonator cavity compared to oil (which has adielectric constant between 2 to 8). This is because the high dielectricconstant of water increases the parasitic capacitance of the resonatorand reduces its resonance frequency. In addition, the high conductivityof the flow (e.g., brine with high salinity) reduces the effectivequality factor of the resonator. This is because the loss of theresonator cavity increases due to conductivity of the flow. Note thathigh conductivity will appear to the EPR sensor as an imaginarycomponent to the dielectric, so that by solving for complexpermittivity, one can obtain both “real” dielectric constant and fluidconductivity.

As noted, the algorithm described above for choosing the EPR frequencymay not output the actual resonant frequency, but rather a frequencywith a reflected signal parameter less than dB0. However, thetheoretical response of the cavity can be readily modelled (e.g.,Rydholm, op. cit.) for a particular (complex) dielectric value. Aminimization algorithm such as Levenberg-Marquadt can then be used tofind that (complex) dielectric value which gives the closest fit to allof the measured data. The modelling can take into account the knownstructure of the resonator (e.g., the dimensions of the sample chamberwithin the pressure housing, the materials for that pressure housing,etc.). The quality factor of the resonator may also be calculated fromthis data since the reflected signal parameter is measured as a functionof the transmitter frequency (as in the graph 1000 of FIG. 10). Thistechnique can be used to build an online flow impedance sensor.

FIG. 11 is a flow diagram of example operations 1100 for determiningboth dielectric permittivity information and an impedance match,according to the workflow described above. The operations 1100 may beperformed, or at least controlled, by a control unit, such as thecontrol unit 902 of FIG. 9.

The operations 1100 may begin, at block 1102, by sweeping over a rangeof frequencies for the RF magnetic field and measuring the reflectedsignals from the resonator cavity. At block 1104, the control unit maysolve for the fluid dielectric constant that gives the best match to themeasured reflected signals. The value of this solution may be stored inmemory (not shown) of the EPR system, which may be connected to or partof the control unit. At block 1106, the control unit may choose afrequency that is at or closest to the frequency selected for a previousEPR sweep and that also gives a reflection parameter value (e.g.,magnitude) that is less than a threshold value (e.g., dB0). At block1108, the values of the operating frequency, the estimated resonantfrequency, and the setting for the impedance match may be stored (e.g.,in memory of the EPR system). The blocks 1102, 1104, 1106, and 1108 maybe repeated, where each iteration through these blocks provides onesetting of frequency and impedance before a DC magnetic sweep isperformed. Alternatively, it is also possible to keep the same impedanceand frequency but perform multiple sweeps of the DC magnetic field. Thiscan be done, for example, to measure multiple EPR spectra, average thesemeasured spectra, and thereby reduce the noise on the output EPRspectrum.

For other aspects, instead of determining the dielectric constant (orcomplex permittivity) directly, it would also be possible to compute theexact resonant frequency and quality factor for each frequency sweep andstore these values.

With regard to the optimizations, or at least adjustments, of theimpedance match and the frequency, these can be done sequentially orconcurrently. Non-limiting examples for the optimization algorithmsinclude gradient descent, quasi-newton, random search, or simulatedannealing.

Different possibilities exist for the timing of the magnetic sweep. Asnoted above, for certain aspects in EPR logging, not only is a DCmagnetic field created, but a small, low frequency modulation is added,where that modulation is done to create a magnetic field parallel to theDC magnetic field. This results in the workflow depicted in FIG. 12.

FIG. 12 is a flow diagram of example operations 1200 for deriving an EPRspectrum using low-frequency magnet modulation, in accordance withcertain aspects of the present disclosure. The operations 1200 may beperformed, or at least controlled, by a control unit, such as thecontrol unit 902 of FIG. 9.

The operations 1200 may begin, at block 1202, by turning on the RF fieldsuch that the RF field is applied to the resonator cavity. At block1204, impedance matching may be performed, and a frequency at or nearresonance may be found. For certain aspects, the control unit may alsoderive permittivity as described above. At block 1206, the control unitmay turn on the DC magnet to the starting point of the magnetic sweep.The low-frequency magnet modulation may be turned on at block 1208. Atblock 1210, the magnetic field may be swept, and the reflected signalsmay be measured. The blocks 1202, 1204, 1206, 1208, and 1210 may berepeated, where each iteration through these blocks provides one EPRspectrum (e.g., as shown in FIG. 13A and described below).

In general, for asphaltene detection, the signal is much smaller thanthat of “background” components, such as metal ions in the fluid, sothere is an interest in zeroing out the background signal to focus onthe asphaltene measurements. For example, FIG. 13A is a graph 1300illustrating an example full EPR response, which shows multiple peaksfrom different resonances. An illustration of an asphaltene signature isshown but, in reality, it may be significantly smaller than thesurrounding peaks, as depicted in the graph 1300. FIG. 13A is a graph1320 illustrating the same curve as FIG. 13A, but in the graph 1320, theDC magnetic field sweep is initiated only over the magnetic intervalpertinent to an asphaltene measurement.

An overall EPR absorption curve would be the sum of individualLorentzian or Gaussian components centered at particular magnetic fieldstrengths, each having a line width dependent on the T1 and T2relaxation values for that particular spin contribution. Because EPRspectrometers modulate the magnetic field, the output of thespectrometer does not look like the sum of such components, however, butis instead the derivative. Thus, the asphaltene signature becomes a“doublet,” as illustrated in the graph 1320 of FIG. 13B.

In some scenarios, it may be desirable to sweep a full magnetic range(e.g., from 0 T to 0.2 T). This is the case, for example, when lookingfor the quantity of some metal ions that have a very broad line width.In other scenarios, it may make sense to sweep a narrower magneticrange. For example, at RF excitations in the 3-5 GHz range, it is knownthat the asphaltene response will occur in the magnetic flux densityinterval of 0.15 T to 0.17 T, so it is completely reasonable to onlysweep that magnetic range (or a range slightly greater than that range).The values 0.15 T and 0.17 T are chosen to demonstrate the example andare not to be taken as limitations of the present disclosure. Also, asmentioned above, the Larmor frequency of an EPR component depends on theratio of DC magnetic field to RF excitation frequency, so if, say, a0.15 T to 0.17 T DC magnetic field sweep is chosen with an RF magneticfield near 4.5 GHz, then a 0.3 T to 0.34 T DC magnetic field sweep wouldbe the appropriate range for an RF magnetic field near 9 GHz. As notedabove, sweeping a narrower magnetic range can allow for a more rapidscan, which can be advantageous when fluids are rapidly flowing. Asdescribed above, a combination of a permanent magnet and anelectromagnet may be used, so in this scenario, an EPR system mayinclude a permanent magnet to provide a magnetic flux density of 0.15 Tand a significantly smaller electromagnet to provide the additionalrange of 0 to 0.2 T. This would allow very rapid sweeping (less than 1second), which may be appropriate for very fast-moving fluids (e.g.,around 30 feet/s).

However, it is also clear from the above example that the EPR responseat 0.15 T is nonzero. This means that there is a nonzero AC magneticsusceptibility as a result of that value of DC magnetic field. If thematching circuit had been set to give minimal impedance mismatch at zerofield (0 T), then there will now be an impedance mismatch unrelated tothe presence of asphaltene in the range of interest therefor.Consequently, instead of making a coupling match with the magnets turnedoff, a coupling match may be made for certain aspects with the magnetset to its initial sweep value for the paramagnetic material of interest(e.g., 0.15 T for asphaltene). This significantly increases thesignal-to-noise ratio (SNR) for the measurement over the narrowed range.

Similarly, for certain aspects, a frequency may be chosen that resonatesnot an empty cavity, nor a simple fluid-filled cavity, but a frequencythat gives minimal reflection when the fluid is undergoing whateverZeeman effect the cavity experiences at the initial magnetic field sweepvalue.

It is also noted that while the Zeeman effect may change μ in thepresence of an applied magnetic field, the same is not true of thepermittivity. Consequently, it is still of use to compute the dielectricand conductivity values from the reflection data at the initial sweep.This gives the workflow of FIG. 14.

FIG. 14 is a flow diagram of example operations 1400 for deriving alocal EPR spectrum by performing impedance and/or frequency sweeps withthe DC magnet activated, in accordance with certain aspects of thepresent disclosure. The operations 1400 may be performed, or at leastcontrolled, by a control unit of an EPR system, such as the control unit902 of FIG. 9.

The operations 1400 may begin, at block 1402, by turning on the RF fieldsuch that the RF field is applied to the resonator cavity. At block1404, the control unit may turn on the DC magnet to the starting pointof the magnetic sweep. The initial value for the magnetic sweep may bebased on the magnetic range for the particular paramagnetic material ofinterest. The low-frequency magnet modulation may be turned on at block1406. At block 1408, impedance matching may be performed, and afrequency at or near resonance may be found, both with the magnetactivated. For certain aspects, the control unit may also derivepermittivity as described above. At block 1410, the magnetic field maybe swept, and the reflected signals may be measured. The blocks 1402,1404, 1406, 1408, and 1410 may be repeated, where each iteration throughthese blocks provides one EPR spectrum (e.g., as shown in FIG. 13B forthe narrowed magnetic sweep).

Additional signal processing may be appropriate, after the narrowermagnetic sweep is complete. For example, one can see from FIG. 13B thata baseline shift may remain on the signal, as indicated by the baseline1322. This shift may be well approximated as a straight line (i.e.,fixed offset plus a slope), and the values of the baseline can becomputed from the spectral values near the beginning and end of thesweep. For other aspects, regression analysis of the spectral values maybe used to calculate the linear equation for the baseline. Effectivelysubtracting out this baseline gives the desired asphaltene response asillustrated in the graph 1500 of FIG. 15, which takes the characteristicform of a doublet (e.g., with a zero slope). In this case, the doubletis the derivative of an absorption spectrum that can be characterized interms of line width, crossing point, and the peak-to-peak voltage (Vpp).Experiments have found good correlation between Vpp and volume ofasphaltene. The line width and crossing point of the doublet correlateto chemical properties of the asphaltene. For example, the line widthmay increase with the geochemical maturity of the asphaltene. Forapplications related to monitoring cleanout and flow assurance, the Vppvalue may be the most useful parameter, together with permittivity orresonance frequency.

As noted above, for certain aspects, the Vpp value can be computed veryquickly (e.g., within a few seconds), so that the final output of theEPR spectrometer becomes a time-series curve of the Vpp values. As aninterface for an operator of the EPR spectrometer, the Vpp value versustime can be plotted directly. As an alternative embodiment, such asshown in the plot 1600 of FIG. 16, the Vpp data can be calibratedagainst the response of reference percentages of asphaltene, in whichcase the data can then be plotted as a curve 1602 representing apercentage of asphaltene versus time (in minutes). Optional additionalcurves added to the same plot 1600 may enhance the value of thespectrometer data. For example, in FIG. 16, a curve 1604 for the cavityresonant frequency (in MHz) has been plotted alongside asphaltenepercentage for the same time series. Additionally or alternatively,other derived values (ϵ, σ, etc.) may also be plotted.

The above description refers specifically to the identification ofasphaltene in crude oil, but a person having ordinary skill in the artwill understand these techniques are equally applicable to any EPRspectrometric analysis where a focused magnetic sweep is performed forthe paramagnetic target of interest.

The measurements of impedance match, resonant frequency, estimateddielectric, and power transmission/absorption may also be used forquality control. Because these measurements are typically made muchfaster than the magnetic sweep is performed, it is possible to take suchmeasurements before and after the magnetic sweep. If the values have notchanged significantly, then one can have confidence that the fluidproperties did not change during the sweep.

The incorporation of fast feedback information into the circuitry of anEPR spectrometer enables continuous EPR logging of a flowing well. Thefeedback allows the enhanced spectroscope to automatically determine anear resonant frequency of a fluid-filled cavity plumbed into a flowline while also updating RF coupling components so as to optimize, or atleast increase, the amplitude of the applied RF magnetic field in theresonator cavity. The values chosen for the excitation frequency and thematching impedance can be incorporated along with the EPR spectrum.

FIG. 17 is a flow diagram of example operations 1700 for performing EPRspectroscopy on a fluid (e.g., from a flowing well), in accordance withcertain aspects of the present disclosure. The operations 1700 may beperformed, or at least controlled, by a control unit, such as thecontrol unit 902 of FIG. 9.

For a first EPR iteration, the operations may begin, at block 1702, byperforming a first frequency sweep of discrete electromagneticfrequencies on a cavity containing the fluid. At block 1704, firstparameter values of reflected signals from the first frequency sweep maybe determined. The control unit may select, at block 1706, a firstdiscrete frequency corresponding to one of the first parameter valuesthat is less than a threshold value. At block 1708, a firstelectromagnetic field may be activated in the fluid at the firstdiscrete frequency. While the first electromagnetic field is activated,a first DC magnetic field sweep may be performed at block 1710 togenerate a first EPR spectrum.

According to certain aspects, the operations 1700 further involveestimating a first resonant frequency of the cavity containing the fluidfor the first EPR iteration. For certain aspects, selecting the firstdiscrete frequency at block 1706 includes selecting one of the discreteelectromagnetic frequencies that is at or closest to the estimated firstresonant frequency as the first discrete frequency. For certain aspects,the operations 1700 may further entail, for a second EPR iterationsubsequent to the first EPR iteration: performing a second frequencysweep of discrete electromagnetic frequencies on the cavity containingthe fluid for the second EPR iteration; determining second parametervalues of reflected signals from the second frequency sweep; comparing aparameter value of a reflected signal in the second frequency sweep tothe threshold value, the parameter value corresponding to the firstdiscrete frequency; activating a second electromagnetic field at thefirst discrete frequency if the parameter value of the reflected signalin the second frequency sweep is less than the threshold value; andwhile the second electromagnetic field is activated, performing a secondDC magnetic field sweep to generate a second EPR spectrum. For certainaspects, the operations 1700 may further include, for the second EPRiteration if the parameter value of the reflected signal in the secondfrequency sweep is greater than the threshold value: estimating a secondresonant frequency of the cavity containing the fluid for the second EPRiteration based on the second frequency sweep; selecting a seconddiscrete frequency; and activating the second electromagnetic field inthe fluid at the second discrete frequency. For certain aspects,selecting the second discrete frequency entails selecting one of thediscrete electromagnetic frequencies that is at or closest to theestimated second resonant frequency as the second discrete frequency.For certain aspects, the parameter value of the reflected signalcomprises a power of the reflected signal, and the threshold value is athreshold power. For other aspects, the parameter value of the reflectedsignal is a voltage amplitude of the reflected signal, and the thresholdvalue comprises a threshold voltage amplitude.

According to certain aspects, the operations 1700 further entailperforming an impedance matching circuit sweep concurrently with theperformance of the first frequency sweep. For other aspects, theimpedance matching circuit sweep may be performed sequentially with theperformance of the first frequency sweep. In either case, the operations1700 may further involve at least one of: determining a quality factor(Q) based on at least one of the first frequency sweep or the impedancesweep; determining a dielectric constant based on at least one of thefirst frequency sweep or the impedance sweep; determining a conductivityof the fluid based on at least one of the first frequency sweep or theimpedance sweep; determining a resonant frequency of the cavity based onat least one of the first frequency sweep or the impedance sweep; ordetermining a composition of the fluid based on at least one of the Q,the dielectric constant, the conductivity, or the resonant frequency.

According to certain aspects, the first frequency sweep is performed inless than 1 ms. For other aspects, the first frequency sweep isperformed in less than 1 s.

According to certain aspects, the cavity containing the fluid is inpressure communication with equipment at a wellsite. In this case, thefluid may be exposed to wellbore (or wellhead) pressure and temperatureduring the performance of the first DC magnetic field sweep.Furthermore, the flowing fluid may not be exposed to extraneous oxygenduring conveyance from the equipment to a resonator having the cavity.

FIG. 18 is a flow diagram of example operations 1800 for performing EPRspectroscopy on a fluid (e.g., from a flowing well), in accordance withcertain aspects of the present disclosure. The fluid may be transportedto a cavity. The operations 1800 may be performed, or at leastcontrolled, by a control unit, such as the control unit 902 of FIG. 9.

The operations 1800 may begin, at block 1802, by activating a magneticfield generator to generate a DC magnetic field at a first magnetic fluxdensity. The magnetic field may be activated in a cavity containing thefluid. While the DC magnetic field is activated at the first magneticflux density, at least one of the following may be performed at block1804: (1) a frequency sweep to determine a frequency for generating aradio frequency (RF) magnetic field in the fluid; or (2) an impedancesweep to set an impedance of an impedance matching circuit associatedwith the generation of the RF magnetic field. At block 1806, theactivated DC magnetic field may be swept from the first magnetic fluxdensity to a second magnetic flux density using at least one of thedetermined frequency or the impedance to generate an EPR spectrum.

According to certain aspects, a range from the first magnetic fluxdensity to the second magnetic flux density corresponds to a particularparamagnetic material of interest. For certain aspects, the particularparamagnetic material of interest is asphaltene. In this case, the firstmagnetic flux density is at least 0.15 T, and the second magnetic fluxdensity is at most 0.17 T. Alternatively, a ratio of the first magneticflux density to the frequency is at least 0.035 T/GHz, and a ratio ofthe second magnetic flux density to the frequency is at most 0.040T/GHz.

According to certain aspects, performing the frequency sweep while theDC magnetic field is activated at the first magnetic flux density atblock 1804 includes determining a resonant frequency of the fluid andselecting a discrete frequency at or closest to the resonant frequencyas the frequency for generating the RF magnetic field.

According to certain aspects, performing the frequency sweep while theDC magnetic field is activated at the first magnetic flux density atblock 1804 entails determining a resonant frequency of the fluid. Inthis case, the operations 1800 may further involve determining a qualityfactor (Q) based on the frequency sweep. For certain aspects, theoperations 1800 may further include at least one of: determining a fluiddielectric constant based on the at least one of the frequency sweep orthe impedance sweep, determining a fluid conductivity based on the atleast one of the frequency sweep or the impedance sweep, determining aresonant frequency of the cavity containing the fluid, or determining acomposition of the fluid based on at least one of the Q, the fluiddielectric constant, the fluid conductivity, or the resonant frequency.

According to certain aspects, the operations 1800 may further involvecalculating a regression line based on the EPR spectrum, removing atleast the slope of the regression line from the EPR spectrum to generatean adjusted EPR spectrum, and determining a peak-to-peak voltage of theadjusted EPR spectrum.

According to certain aspects, the operations 1800 may further entailrepeating the performing at block 1804 and the sweeping at block 1806 togenerate a plurality of EPR spectrums; storing at least one of thefrequency for each frequency sweep or a parameter indicative of theimpedance setting for each impedance sweep; determining a plurality ofpeak-to-peak voltages based on the plurality of EPR spectrums; andplotting the plurality of peak-to-peak voltages versus time and the atleast one of the stored frequencies or the stored impedances versustime. For example, the parameter indicative of the impedance setting maybe a voltage for a varactor (or a digital value for a switched capacitoror switched array of capacitors) in the impedance matching circuit.

According to certain aspects, the operations 1800 may further includeafter the sweeping at block 1806, performing at least one of anotherfrequency sweep to determine at least one of another frequency oranother impedance sweep to set another impedance; comparing at least oneof the frequency with the other frequency or the impedance with theother impedance; and determining whether one or more properties of thefluid changed during the sweeping of the activated DC magnetic fieldbased on the comparison.

According to certain aspects, a cavity containing the fluid is inpressure communication with equipment at a wellsite. In this case, thefluid may be exposed to wellhead (or wellbore) pressure and temperatureduring the sweeping of the activated DC magnetic field. Furthermore, thefluid may not be exposed to extraneous oxygen during conveyance from theequipment to a resonator having the cavity.

Certain aspects of the present disclosure provide an EPR spectrometerfor performing EPR spectroscopy on a fluid from a flowing well. The EPRspectrometer generally includes a tube capable of receiving the fluid; amagnetic field generator configured to generate a DC magnetic field inthe fluid during operation of the EPR spectrometer; transmit circuitryconfigured to generate a radio frequency (RF) signal; a resonatorcoupled to the transmit circuitry and configured to convert the RFsignal into an RF magnetic field in the fluid during the operation ofthe EPR spectrometer; impedance matching circuitry coupled between thetransmit circuitry and the resonator; receive circuitry configured toreceive and process reflected signals from the fluid via the resonator;and at least one processor coupled to the magnetic field generator, thetransmit circuitry, and the receive circuitry. The at least oneprocessor is generally configured to activate the magnetic fieldgenerator to generate the DC magnetic field at a first magnetic fluxdensity; to control, while the DC magnetic field is activated at thefirst magnetic flux density, at least one of: (1) the transmit circuitryto perform a frequency sweep to determine a frequency for generating theRF magnetic field in the fluid; or (2) the impedance matching circuitryto perform an impedance sweep to set an impedance of the impedancematching circuitry associated with the generation of the RF magneticfield; and to control the magnetic field generator to sweep theactivated DC magnetic field from the first magnetic flux density to asecond magnetic flux density using at least one of the determinedfrequency or the impedance to generate an EPR spectrum.

According to certain aspects, a range from the first magnetic fluxdensity to the second magnetic flux density corresponds to a particularparamagnetic material of interest. For example, the particularparamagnetic material of interest is asphaltene. In this case, the firstmagnetic flux density may be 0.15 T, and the second magnetic fluxdensity may be 0.17 T (e.g., for the determined frequency in a rangefrom 3 to 5 GHz).

According to certain aspects, the at least one processor is configuredto control the transmit circuitry to perform the frequency sweep whilethe DC magnetic field is activated at the first magnetic flux density bydetermining a resonant frequency of the fluid and selecting a discretefrequency at or closest to the resonant frequency as the frequency forgenerating the RF magnetic field.

According to certain aspects, the at least one processor is configuredto control the transmit circuitry to perform the frequency sweep whilethe DC magnetic field is activated at the first magnetic flux density bydetermining a resonant frequency of the fluid. In this case, the atleast one processor may be further configured to determine a qualityfactor (Q) based on the frequency sweep. For certain aspects, the atleast one processor is further configured to determine a dielectricconstant based on the resonant frequency; to determine a conductivity ofthe fluid based on the quality factor; to calculate a complexpermittivity based on the dielectric constant and the conductivity; andto determine a composition of the fluid based on the complexpermittivity.

According to certain aspects, the at least one processor is furtherconfigured to calculate a baseline (e.g., a regression line) based onthe EPR spectrum, to remove at least the slope of the baseline from theEPR spectrum to generate an adjusted EPR spectrum, and to determine apeak-to-peak voltage of the adjusted EPR spectrum.

In one aspect, components of an enhanced EPR spectrometer may comprise aresonator surrounding a cylindrical cavity through which pressurizedwell-fluids can flow, an RF transceiver with at least one variablecomponent to couple RF energy to the resonator, a mechanism to measurethe efficiency of the coupling, a magnetic field generator withadjustable magnetic field, and a feedback loop to update the couplingbased on the properties of the fluid as the fluid enters the cavity. Thespectrometer may use a circulator to measure the reflected microwavepower from the resonator.

Further, the resonator may be a loop-gap resonator with a loopperpendicular to the cavity axis. The magnetic field generator mayproduce a magnetic field, more particularly a DC magnetic field. Themagnetic field generator may comprise magnets, coils, or a combinationthereof that are positioned exterior to the cavity to generate amagnetic field therein. The transceiver of the EPR spectrometer mayprovide a pulsed or continuous signal to the resonator to generate apulsed or continuous RF magnetic field approximately normal to the DCmagnetic field of the magnets or coils. A receiver of the EPRspectrometer may monitor a reflected signal to detect changes inreflected signal. The frequency of RF transmission may be changed as maybe the variable component in the coupling from the transceiver to theresonator. These changes may be changed as part of individual feedbackloops or simultaneously as part of a dual feedback loop. The RFfrequency may be chosen to be near the resonance frequency of thecavity. The strength of the DC magnetic field may be swept. A modulationfrequency may be further superimposed on that sweep. The initiation ofthat magnetic field sweep may be triggered by a range of values of thechosen frequency or coupling component. The changes in the reflectedsignal based on the changing DC field, the RF frequency, and thecoupling component may be analyzed to determine the materials presentand material concentrations.

Any of the operations described above, such as the operations 1100,1200, 1400, 1700, and/or 1800 may be included as instructions in acomputer-readable medium for execution by a control unit (e.g., controlunit 902 or controller module 706) or any other processing system. Thecomputer-readable medium may comprise any suitable memory for storinginstructions, such as read-only memory (ROM), random access memory(RAM), flash memory, an electrically erasable programmable ROM (EEPROM),a compact disc ROM (CD-ROM), a floppy disk, and the like.

While the foregoing is directed to certain aspects of the presentdisclosure, other and further aspects may be devised without departingfrom the basic scope thereof, and the scope thereof is determined by theclaims that follow.

The invention claimed is:
 1. A method of monitoring tracer particlesconveyed by a fluid from a flowing well penetrating a reservoir, themethod comprising, for a first EPR iteration: exposing the fluid to afirst frequency sweep of an electromagnetic field wherein the firstfrequency sweep is over a set of discrete electromagnetic frequencies;determining first parameter values of reflected signals from the firstfrequency sweep; selecting a first discrete frequency corresponding toone of the first parameter values that is less than a threshold value;activating a first electromagnetic field in the fluid at the firstdiscrete frequency; performing a first DC magnetic field sweep togenerate a first EPR spectrum while the first electromagnetic field isactivated; and deriving a property of the conveyed tracer particles fromthe first EPR spectrum.
 2. The method of claim 1, wherein the conveyedtracer particles comprise magnetic or paramagnetic nano-particles. 3.The method of claim 1, wherein the property of the conveyed tracerparticles is a concentration of the conveyed tracer particles within thefluid.
 4. The method of claim 1, wherein the property of the conveyedtracer particles comprises one or more paramagnetic properties of theconveyed tracer particles within the fluid.
 5. The method of claim 1,wherein the conveyed tracer particles comprise two or more differentcategories of particles and wherein the property of the conveyed tracerparticles comprises a relative concentration of one category to another.6. The method of claim 1, wherein exposing the fluid to theelectromagnetic field comprises: flowing a first sample of the fluidinto a cavity; and exposing the fluid to the first electromagnetic fieldafter the first sample has entered the cavity.
 7. The method of claim 6,further comprising, for a second EPR iteration subsequent to the firstEPR iteration: flowing a second sample of the fluid into the cavity;performing a second frequency sweep of discrete electromagneticfrequencies on the second sample of the fluid for the second EPRiteration; determining second parameter values of reflected signals fromthe second frequency sweep; comparing a parameter value of a reflectedsignal in the second frequency sweep to the threshold value, theparameter value corresponding to the first discrete frequency;activating a second electromagnetic field at the first discretefrequency if the parameter value of the reflected signal in the secondfrequency sweep is less than the threshold value; performing a second DCmagnetic field sweep while the second electromagnetic field is activatedto generate a second EPR spectrum; and deriving properties of theconveyed tracer particles in the second sample from the second EPRspectrum.
 8. The method of claim 7, further comprising: performing amultiplicity of EPR iterations; and determining a baseline propertyvalue of the conveyed tracer particles.
 9. The method of claim 1,further comprising injecting the tracer particles into the reservoir,wherein the injected tracer particles are subsequently conveyed by thefluid from the flowing well.
 10. The method of claim 9, wherein thetracer particles are injected in a first well and wherein an EPRspectrum is derived from fluid flowing from the first well.
 11. Themethod of claim 9, wherein the tracer particles are injected in a firstwell and wherein an EPR spectrum is derived from fluid flowing from asecond well.
 12. A non-transitory computer-readable medium storinginstructions that, when executed on a processor, perform operations fordetecting a property of a fluid flowing through a system, the operationscomprising: exposing the fluid to a frequency sweep, wherein thefrequency sweep is over a set of discrete electromagnetic frequencies;determining parameter values of reflected signals from the frequencysweep; selecting a discrete frequency corresponding to one of theparameter values that is less than a threshold value; activating anelectromagnetic field in the fluid at the discrete frequency; performinga DC magnetic field sweep to generate an EPR spectrum while theelectromagnetic field is activated; and deriving a property of tracerparticles conveyed by the fluid from the EPR spectrum.
 13. An apparatusfor monitoring tracer particles conveyed by a fluid from a flowing wellpenetrating a reservoir, comprising: a tube having a cavity capable ofreceiving the fluid comprising the tracer particles; an electronparamagnetic resonance (EPR) spectrometer, the EPR spectrometer beingconfigured to perform EPR spectroscopy on at least a portion of thefluid to generate an EPR spectrum; a magnetic field generator configuredto generate a DC magnetic field in the fluid during operation of the EPRspectrometer; transmit circuitry configured to generate a radiofrequency (RF) signal; a resonator coupled to the transmit circuitry andconfigured to convert the RF signal into an RF magnetic field in thefluid during the operation of the EPR spectrometer; receive circuitryconfigured to receive and process reflected signals from the fluid viathe resonator; and at least one processor coupled to the magnetic fieldgenerator, the transmit circuitry, and the receive circuitry andconfigured, for a first EPR iteration, to: control the transmitcircuitry to perform a first frequency sweep of discrete electromagneticfrequencies on the cavity containing the fluid; determine firstparameter values of reflected signals from the first frequency sweep;select a first discrete frequency corresponding to one of the firstparameter values that is less than a threshold value; control thetransmit circuitry to activate a first electromagnetic field in thefluid at the first discrete frequency; and control the magnetic fieldgenerator, while the first electromagnetic field is activated, toperform a first DC magnetic field sweep to generate a first EPRspectrum.
 14. The apparatus of claim 13, wherein the at least oneprocessor is further configured to estimate a first resonant frequencyof the cavity containing the fluid for the first EPR iteration.
 15. Theapparatus of claim 14, wherein the at least one processor is configuredto select the first discrete frequency by selecting, as the firstdiscrete frequency, one of one or more discrete electromagneticfrequencies that is at or closest to the estimated first resonantfrequency.
 16. The apparatus of claim 13, wherein the at least oneprocessor is further configured, for a second EPR iteration subsequentto the first EPR iteration, to: control the transmit circuitry toperform a second frequency sweep of discrete electromagnetic frequencieson the cavity containing the fluid for the second EPR iteration;determine second parameter values of reflected signals from the secondfrequency sweep; compare a parameter value of a reflected signal in thesecond frequency sweep to the threshold value, the parameter valuecorresponding to the first discrete frequency; control the transmitcircuitry to activate a second electromagnetic field at the firstdiscrete frequency if the parameter value of the reflected signal in thesecond frequency sweep is less than the threshold value; and control themagnetic field generator, while the second electromagnetic field isactivated, to perform a second DC magnetic field sweep to generate asecond EPR spectrum.